Field of the Disclosure
The present disclosure generally relates to a riserless well abandonment operation using sealant and cement.
Description of the Related Art
FIGS. 1A-1C illustrate a prior art completed subsea well. A portion of a conductor string 3 is driven into a floor 1f of the sea 1. The conductor string 3 includes a housing 3h generally extending above the seafloor and joints of conductor pipe 3p connected together, such as by threaded connections, extending into the sea floor. Once the conductor string 3 has been set, i.e., the joints of conductor pipe 3p driven into the sea floor 1f, a subsea wellbore 2 is drilled into the seafloor 1f through the conductor pipe 3 and extended into one or more subsurface formations 9u. A surface casing string 4 is deployed into the conductor string 3. The surface casing string 4 commonly includes a wellhead housing 4h supported on the housing 3h and joints of casing 4c connected together, using, for example, threaded connections, and extending inwardly of the conductor pipe 3p. The wellhead housing 4h lands in the conductor housing 3h during deployment of the surface casing string 4. Cement 8s is used to secure the surface casing string 4 in the wellbore 2 within the conductor pipe 3p. Once the surface casing string 2 is set, the wellbore 2 is further extended (drilled into) and an intermediate casing string 5 is then deployed into the wellbore. The intermediate casing string 5 commonly includes a hanger 5h and joints of casing 5c connected together, using, for example, threaded connections. Cement 8i is used to secure the intermediate casing string 5 in the wellbore 2 and seal of the space between the intermediate casing string 5 and the adjacent surface of the drilled borehole. The hanger 5h of the intermediate casing string 5 is supported in the wellhead housing 4h. 
Once the intermediate casing string 5 has been set, the wellbore 2 is extended into (drilled into) a hydrocarbon-bearing (i.e., crude oil and/or natural gas) reservoir 9r. The production casing string 6 is then deployed into the wellbore. The production casing string 6 includes a hanger 6h supported on the hanger 5h of the intermediate casing string, and joints of casing 6c connected together, using, for example, threaded connections, extending therefrom through the intermediate casing string 5. Cement 8p is used to secure the production casing string 6 in the wellbore 2 and seal of the annular region between the production casing string 6 and the wall of the wellbore 2, at a location lower in the well than that of cement 8i. Each casing hanger 5h, 6h is sealed off in the wellhead housing 4h by a packoff. The housings 3h, 4h and hangers 5h, 6h are collectively referred to as a wellhead 10.
A production tree 15 is connected to the wellhead 10, such as by a tree connector 13. The tree connector 13 includes a fastening device, such as dogs, for fastening the tree to an external profile of the wellhead 10. The tree connector 13 further includes a hydraulic actuator and an interface, such as a hot stab, so that a remotely operated subsea vehicle (ROV) 20 (FIG. 2A) can operate the actuator for engaging the dogs with the external profile. The tree 15 is vertical or horizontal. If the tree is vertical (not shown), it is installed after a production tubing string 7 is hung from the wellhead 10. If the tree 15 is horizontal (as shown), the tree is installed and then the production tubing string 7 is hung from the tree 15. The tree 15 includes fittings and valves to control production from the wellbore 2 into a pipeline (not shown) which may lead to a production facility (not shown), such as a production vessel or platform.
The production tubing string 7 includes a hanger 7h and joints of production tubing 7t connected together, such as by threaded connections. The production tubing string 7 includes a subsurface safety valve (SSV) 7v interconnected with the tubing joints 7t and a hydraulic conduit 7c extending from the valve 7v to the hanger 7h as shown in FIG. 1B. The production tubing string 7 further includes a production packer 7p and the packer is set between the lower end of the production tubing string 7 and the production casing string 6 directly adjacent to the lower end of the production tubing to isolate an annulus 7a (aka the A annulus) formed therebetween from production fluid (not shown). The tree 15 is also in fluid communication with the hydraulic conduit 7c. A portion of the production casing string 6 is perforated by perforations 11 as shown in FIG. 1C, which are formed using a perforation tool to provide fluid communication between the reservoir 9r and a bore of the production tubing string 7. The production tubing string 7 is configured to transport production fluid from the reservoir 9r to the production tree 15.
The tree 15 includes a head 12, the tubing hanger 7h, the tree connector 13, an internal cap 14, an external cap 16, an upper crown plug 17u, a lower crown plug 17b, a production valve 18p, one or more annulus valves 18u,b, and a face seal 19. The tree head 12, tubing hanger 7h, and internal cap 14 each have a longitudinal bore extending therethrough. The tubing hanger 7h and head 12 each have a lateral production passage formed through walls thereof for the flow of production fluid therethrough. The tubing hanger 7h is disposed in the head 12 bore. The tubing hanger 7h is fastened to the head 12 by a latch.
Once the reservoir 9r is produced to depletion or is not feasible to produce or continue producing therefrom, the well may be abandoned. Conventionally, an abandonment operation includes cutting into the casings, and filling the annuli between the casing strings and the wellbore 2 wall with cement to seal the upper regions of the annuli. To achieve this, it is usual to use a semi-submersible drilling vessel (SSDV) which is located above the well and anchored in position. After removal of the cap 16 from the well, a unit including blow-out preventers and a riser is lowered and locked on to the wellhead. A tool string is run-in on pipe to sever or perforate the casing or casings. Weighted fluid is pumped into the well to provide a hydrostatic head to balance any possible pressure release when the casing is cut. The casing is then cut, and the annulus cemented. The cemented annulus is then pressure tested to ensure that an adequate seal between the casings and the wellbore 2 wall has been obtained. The casing is severed below the mud line and the casing hangers retrieved, and finally after removal all removable equipment is removed from the well, the well is filled with cement. Whilst by this procedure satisfactory well abandonment can be achieved, it is expensive in terms of the equipment involved and the time taken which is often from seven to ten days per well.
Historically, Portland cement has served as the standard for sealing the casing annulus for abandonment. However, Portland cement properties, both unset and set, are not ideal for creating a durable seal. The Portland cement slurry is aqueous and will dilute when intermixed with water present in the well. The set Portland cement is brittle and could fail over time. Therefore, a more durable sealant and seal are desired.